Background
Mutational inactivation of the SETDB1 histone methyltransferase is found in a subset of mesothelioma, particularly in cases with near-haploidy and TP53 mutations. However, the tumourigenic consequences of SETDB1 inactivation are poorly understood.
Conclusion
These findings demonstrate that SETDB1 inactivation in near-haploid mesothelioma is generally associated with complete loss of SETDB1 protein expression and dysregulates TP53 expression. Targeting SETDB1 pathways could be an effective therapeutic strategy in these often untreatable tumours.
Methods
In this study, we investigated SETDB1 tumour suppressor functions in mesothelioma and explored biologic relationships between SETDB1 and TP53.
Results
Immunoblotting of early passage cultures showed that SETDB1 was undetectable in 7 of 8 near-haploid mesotheliomas whereas SETDB1 expression was retained in each of 13 near-diploid mesotheliomas. TP53 aberrations were present in 5 of 8 near-haploid mesotheliomas compared to 2 of 13 near-diploid mesotheliomas, and BAP1 inactivation was demonstrated only in near-diploid mesotheliomas, indicating that near-haploid and near-diploid mesothelioma have distinct molecular and biologic profiles. Lentiviral SETDB1 restoration in near-haploid mesotheliomas (MESO257 and MESO542) reduced cell viability, colony formation, reactive oxygen species levels, proliferative marker cyclin A expression, and inhibited growth of MESO542 xenografts. The combination of SETDB1 restoration with pemetrexed and/or cisplatin treatment additively inhibited tumour growth in vitro and in vivo. Furthermore, SETDB1 restoration upregulated TP53 expression in MESO542 and MESO257, whereas SETDB1 knockdown inhibited mutant TP53 expression in JMN1B near-haploid mesothelioma cells. Likewise, TP53 knockdown inhibited SETDB1 expression. Similarly, immunoblotting evaluations of ten near-diploid mesothelioma biopsies and analysis of TCGA expression profiles showed that SETDB1 expression levels paralleled TP53 expression.